Viscous material, such as mastic caulking material, is commonly supplied in a cartridge having a discharge nozzle. The cartridge can be mounted in a dispensing appliance, often called a dispensing gun or applicator. An example of such a dispensing gun is described in British patent GB1555455. The gun has a plunger or a push rod slideably mounted in a cartridge. The cartridge is mounted in a keep before the plunger. The plunger is advanced by means of a catch plate linked to a trigger lever engaging the rod. When a dispensing force is applied to the trigger lever, the plunger forces a piston, inside one end of the cartridge, forward to urge the material from the nozzle at the other end. The trigger and catch plate are reset at the end of a dispensing stroke and the dispensing force can be reapplied.
Between trigger strokes the plunger is held against the piston by means of a locking plate. In order to retreat the plunger, for example to remove the cartridge, a force is applied to the locking plate to disengage the locking plate from the rod and allow retreat of the rod. The locking plate can be arranged to either substantially prevent any retreat of the rod during dispensing or to allow a small amount of retreat of the rod to release pressure applied to the cartridge, as disclosed, for example, in European patent EP0448375. In order to be able to retreat the plunger, the locking plate must be accessible so that a releasing force can be applied to it, either directly or by an intermediate mechanical linkage where the locking plate is within a housing together with the remaining actuating mechanism, for example as disclosed in U.S. Pat. No. 5,370,282.
The inventors have realized that, for a number of reasons, it may be undesirable to require access to the locking plate or a corresponding release lever or button in order to release the locking plate to allow retreat of the rod. For example, if the locking plate or lever is arranged such that it can be actuated by the thumb of a user's hand holding the gun to leave the other hand free to retreat the rod, this will compromise the design freedom in designing the gun, in particular for dispensing guns of overall small size. Similarly, the need for an external locking plate or lever tends to make the dispensing gun more difficult to clean, even if the remaining actuating mechanism is enclosed in a housing. These considerations are particularly relevant for dentistry applications, where the nature of the work and the small quantities of material to be dispensed means that the dispenser is likely to be of a small form factor and where the ease of maintaining the dispensing gun cleanliness is paramount. However, it will be understood that these considerations are not limited to dentistry applications.